Pickling zirconium and zirconium base alloys



March 7, 1964 'R. M. WATKINS ETAL 3,125,474

PICKLING ZIRCONIUM AND ZIRCONIUM BASE ALLOYS Filed Jan. 6/1960 z llImmersion Time -Minutes Fig. I

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e Wh .0 MC 0 fl e0 bnY 008 RR ITNESSES United States Patent Filed Jan.6, 1960, er. No. 797 9 Claims. (Cl. 156-18) The present inventionrelates to improved etching and pickling solutions and a process forpickling zirconium and zirconium base alloys.

In the presently accepted procedure for the fabrication of zirconium andzirconium base structural members especially for applications such asreactor components, mechanical operations, such as machining the alloygives rise to a layer of disturbed metal at the surface which extendsfor an appreciable depth. As this disturbed layer exhibits a more rapidcorrosion rate than does the bulk metal, it is removed by chemicaldissolution.

A known method of surface pickling consists of the removal of from 1 to2 mils from the surfaces of zirconium alloy members by bright etching inan aqueous solution containing predetermined proportions by volume ofnitric acid and hydrofluoric acid. This admittedly hazardous reagent iscapable of producing severe burns upon even momentary skin contact, andis contained in brick tanks with polyvinyl chloride liners as it evenattacks stainless steel at a rapid rate. In the majority of processesinvolving zirconium and zirconium base alloys in which the alloy is tobe pickled, a large ratio of etchant volume to component surface area isdesired as the zirconium-acid reaction is exothermic, and the etchanttemperature should be maintained below 120 F. Items pickled attemperatures in excess of the upper limit result in tapered andirregular surfaces and are more prone to be contaminated.

Contamination occurs when the etchant is not thoroughly rinsed from thesurfaces or when the etchant is permitted to dry on the member. In thelatter event, the residue is insoluble in water. Inadequate rinsing,which allows some of the etchant to dry on the surface, leaves the alloywith poor corrosion resistance and subsequently it leads to theformation of undesirable white. corrosion products when exposed to hightemperature water.

The difiiculties which could result from this condition, namely theoccurrence of white corrosion products, are several. First, the whitecorrosion product could be evidence of a sufficiently acceleratedfluoride attack to significantly reduce the thickness of the zirconiumand its alloys. Second, a bulky corrosion product would reduce the heattransfer coefficient. Third, a non-adherent zirconium oxide product,especially in nuclear reactors, could be carried away by a coolant andthus increase the contamination and accessibility problem.

Summing up, the present state of the art of zirconium and zirconium basealloy surface preparation involves the use of the hazardoushydrofluoric-nitric acid etchant procedure with its stringent rinserequirements, diflicult contaminant problems, and critical temperaturecontrol for maintenance of dimensional tolerances and questionableover-all efliciency.

In pickling zirconium and zirconium base alloys, using modifications ofthe standard hydrofluoric nitric acid procedure, it has been observedthat the higher the percentage of nitric acid in the-reagent, the slowerthe rate of reaction. Hydrofluoric acid alone produces a violentexothermic reaction with zirconium and its alloys, whereas nitric acidalone does not attack the alloy. Apparently the nitric acid acts as aninhibitor and serves to control the reaction rate.

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Some work has been done on the substitution of ammonium fluoride for thehydrofluoric acid in dissolving formulations recommended for applicationin decladding zirconium and zirconium base reactor fuel elements. Theseformulations are ineffectual in the controlled pickling of zirconiumbase alloys containing tin as they are incapable of dissolving certaincomponents of the alloy which are believed to be tin and zirconiumhydride. These appear as a black deposit on the metal surface during thedissolving process and result in poor surface finish quality.

Therefore, it is an object of the present invention to provide anaqueous etching and pickling solution comprising predeterminedproportions of ammonium persulfate, hydrofluoric acid and/ or ammoniumbilluoride, with or without acetic acid and/ or ammonium nitrate.

It is another object of the present invention to provide an aqueousetching and pickling solution for zirconium and zirconium base alloyscomprising predetermined proportions of ammonium persulfate, hydrogenfluoride and/ or ammonium bifluoride, with or without acetic acid and/or ammonium nitrate.

It is still another object of the invention to provide a process foretching and pickling zirconium and zirconium base alloys in aqueoussolutions comprising predetermined proportions of ammonium persulfate,hydrofluoric acid or ammonium bifluoride, with or without acetic acidand/ or ammonium nitrate.

Other objects will, in part, be obvious and will, in part, appearhereinafter.

Reference should be made to the following detailed description anddrawing, in which the two figures are graphs plotting time againstsurface thickness dissolved.

In accordance with the present invention, there is provided an improvedetching and pickling solution for zirconium and zirconium base alloys.The solution comprises predetermined proportions of ammonium persulfate,hydrofluoric acid and/ or ammonium bifluoride, with or without aceticacid and/or ammonium nitrate. The solution is characterized by a slow,linear pickling rate with better control of surface dissolution andreduced stringent rinse requirements to avoid staining. Members treatedwith the solution have high surface finish quality. Furthermore, thesolutions will result in lower pickling tank cost by substitution of analloy such as stainless steel for the container or tank material inplace of the brick, polyvinyl chloride lined tanks now required forpickling of zirconium and its alloys.

The pickling solution of this invention may comprise an aqueous solutionof (1) either .5 to 5% by weight hydrofluoric acid or from .5% to 8% byWeight ammonium bifiuoride, or both, the amount of hydrogen fluoridesupplied by mixtures of ammonium biiluoride and hydrofluoric acidtotaling at least 0.3% and not in excess of 3% and (2) from 3% to 30% byWeight ammonium persulfate. In addition, there may be present eitheracetic acid or ammonium nitrate. Acetic acid is highly desirable inamounts of from 3% to 20% by weight and ammonium nitrate in amounts offrom 2% to 6%.

The etching and pickling solution may be prepared in a resistantcontainer and mixed by any suitable means. It is preferred to mix thesolution in a tank of stainless steel, or of another inert material,taking care that the ammonium persulfate is dissolved first in the Waterto protect the stainless steel tank from fluoride attack when thefluoride component is added. The ammonium persulfate functions as aninhibitor of fluoride attack on metallic zirconium in the picklingprocess. The acetic acid and/ or ammonium nitrate, while they are notnecessary constituents in the pickling solution, are highly desirablesince the acetic acid inhibits hydrogen embrittlement of zirconium whilethe ammonium nitrate, in conenema/a junction with ammonium persulfate,insures a complete solution of any tin and zirconium hydride remainingon the surface of the zirconium base alloys containing tin.

Zirconium and zirconium base alloy members may be etched or pickled bysuspending the members on a fixture laterally disposed across theopening of the pickling tank containing the pickling solution. Thefixture and the tank may be composed of an alloy such as stainless steelor other inert metals, the dimensions of the same being determined bythe dimensions of the members being pickled. 'Ilhe pickling solutionalso may be applied to members of the alloy by any suitable means, suchas hosing or spraying. The solution is preferably maintained at atemperature of from 70 to 120 F. The time in solution is regulated bythe desired depth of surface dissolved with which it has a linearrelationship.

It may be desirable in some cases to modify the action of the picklingsolution on the zirconium and zirconium base alloy members by inducingan electrical current in the members. The members may be made the anode,and the cathode may be composed of any suitable material, such asgraphite or platinum. The surface dissolution of the members isfacilitated by electrolysis. However, it should be understood that it isnot necessary to induce an electrical current in the members to achievethe results as described in the present invention.

The surfaces produced by the solution of these processes appear brightand of a quality equal to the surfaces produced by means of the standardnitric-hydrm fluoric acid pickling procedure. In addition, the attackproduced by the pickling solutions of the present invert tion is easilycontrollable and appears to be linear with time, as is illustrated byFIGS. 1 and 2.

:In the following examples, six sample coupons (.7" x .7" x. .08) eachWeighing approximately 4 grams were sectioned from an alloy member andeach was pickled in a pickling solution, five of which differed incomposition, each solution being initially at room temperature. Thealloy comprised approximately 1.5% by weight of tin, 0.1% by weight ofchromium, 0.05% by weight of nickel, 0.1% by weight of iron and thebalance being zirconium. All solution percentages used are by weight.

Example I A sample couplon Was immersed in an aqueous pickling solutioncomprising 2.5% ammonium biflouride, 4% ammonium nitrate, and ammoniumpersulfate for a period of 20 minutes. The coupon surfaces were shinyand uniform in dimensions.

Referring to FIG. 1, there is a graph plotting surface thicknessdissolved over a period of time. The sample was measured at frequentintervals and shows a straight line relationship.

Example I] A sample coupon was immersed in an aqueous pickling solutioncomprising 1.7% by weight hydrogen fluoride (added as 50% HF solution),7% glacial acetic acid and 20% ammonium persulfate for a period of 30minutes. The surfaces also appeared shiny and uniform in dimensions.

Referring to FIG. 2, there is a graph plotting surface thicknessdissolved over a period of time. The sample was measured at frequentintervals and shows a straight line relationship.

Example III (Prior Art) A sample coupon was immersed in an aqueouspickling solution comprising by volume, 39% nitric acid (sp. gr. 1.42)and 3% hydrofluoric acid (52% by weight aqueous solution) for a periodof 5 minutes. The coupon appeared dull and rough surfaced.

Another sample coupon was immersed in aqueous solution of the samecomposition of this Example III, for a period of 5 minutes while thesolution was being vigorously agitated. The pickling reaction was veryrapid and uncontrollable. The resulting coupon appeared bright and shinybut the surfaces became very irregular due to the rapid fluoride attack.Both coupons evidenced a substantial weight loss.

In order to illustrate the criticality of the presence of appreciableamounts of ammonium persulfate in the compositions of this invention,the following solutions were prepared without ammonium persulfate.

Example IV A sample coupon was immersed in an aqueous pickling solutioncomprising 3% ammonium fluoride for a period of 5 minutes with constantagitation. The surfaces of the coupon were covered with a black coating,believed to be tin and/or zirconium hydride.

Example V A sample coupon was immersed in an aqueous pickling solutioncomprising 2.5 ammonium fluoride and 4% ammonium nitrate for a period of5 minutes with constant agitation. The surface of the treated couponappeared coarse, stained and somewhat pitted.

The above described pickling solutions of the invention can be employedto pickle, with equally good results to those set forth in examples 1and 2, zirconium metal and members of an alloy consisting essentially offrom 0.1% to 2.5% by weight of tin, a total of from 0.1% to 2% by weightof at least one metal from period 3 of the periodic table selected fromthe group consisting of iron, nickel and chromium, with or without from0.03% to 1% by weight of beryllium, and the balance zirconium.

It is to be understood that the above description should be interpretedas being illustrative and not limiting.

We claim as our invention:

1. An aqueous etching and pickling solution suitable for zirconium andzirconium base alloys consisting essentially of from 3% to 30% by Weightammonium persulfate to inhibit fluoride attack on zirconium, at leastone of the group acetic acid and ammonium nitrate in the solution, therebeing up to 20% by weight acetic acid, and up to 6% by weight ammoniumnitrate, the acetic acid being employed to inhibit hydrogenembrittlement of zirconium and the ammonium nitrate being employed toinsure a complete solution of any tin or zirconium hydride remaining onthe surface of a zirconium base alloy containing tin, and at least onehydrogen fluoride containing compound selected from the group consistingof hydrofluoric acid and ammonium bifluoride in an amount to provide atleast 0.3% and not more than 3% by weight of hydrogen fluoride.

2. An aqueous etching and pickling solution suitable for etchingzirconium and zirconium base alloys consisting essentially of, byweight, from 0.3 to 3% hydrogen fluoride, not exceeding 20% of aceticacid and 3% to 30% ammonium persultfate.

3. An aqueous etching and pickling solution suitable for etchingzirconium and zirconium base alloys consisting essentially of, byweight, from 0.5% to 8% ammonium bifluoride, not exceeding 6% ofammonium nitrate and 3% to 30% ammonium persulfate.

4. An aqueous etching and pickling solution suitable for etchingzirconium and zirconium base alloys consisting essentially of, byweight, 1.7% hydrogen fluoride, 7% acetic acid and 20% ammoniumpersulfate.

5. An aqueous etching and pickling solution suitable for etchingzirconium and zirconium base alloys consisting essentially of, byweight, 2.5% ammonium hydrogen fluoride, 4% ammonium nitrate and 15%ammonium persulfate.

6. The process of pickling zirconium and zirconium base alloys,comprising, applying to members of the alloy an aqueous picklingsolution consisting essentially of from 3% to 30% by weight ammoniumpersulfate at least one hydrogen fluoride containing compound selectedfrom the group consisting of hydrofluoric acid and ammonium bifluoridein an amount to provide at least 0.3% and not more than 3% by weighthydrogen fluoride for a. period of time at a temperature of from 70 F.to 120 F.

7. The process of pickling zirconium and zirconium base alloys,comprising, applying to members of the alloy an aqueous picklingsolution consisting essentially of, by weight, from 0.3% to 3% hydrogenfluoride, not exceeding 20% of acetic acid, and 3% to 30% ammoniumpersultfate for a period of time at a temperature of from 70 F. to 120F.

8. The process of pickling zirconium and zirconium base alloys,comprising, applying to the members of the alloy an aqueous picklingsolution consisting essentially of, by weight, from 0.3% to 3% ammoniumbifluoride, not exceeding 6% of ammonium nitrate, and 3% to 30% ammoniumpersulfate for a period of time at a temperature of from 70 F. to 120 F.

9. The process of pickling an alloy consisting essentially of from 0.1%to 2.5% by Weight of tin, a total of from 0.1% to 2% by weight of atleast one metal from the group consisting of iron, nickel, and chromium,from 0.03% to 1% by weight of beryllium, and the balance zirconium,comprising applying to the members of the alloy an aqueous picklingsolution consisting essentially of, by Weight, from 0.3% to 3% hydrogenfluoride, not exceeding 20% of acetic acid up to 6% ammonium nitrate and3% to ammonium persulfate for a period of time at a temperature of fromF. to F.

References Cited in the file of this patent UNITED STATES PATENTS1,969,678 White Aug. 7, 1934 2,678,876 Burnside May 18, 1954 2,684,892Saulnier July 27, 1954 2,856,275 Otto Oct. 14, 1958 2,942,954 ThomasJune 28, 1960 2,981,610 Synder et al. Apr. 25, 1961

7. THE PROCESS OF PICKLING ZIRCONIUMAND ZIRCONIUM BASE ALLOYS,COMPRISING, APPLYING TO MEMBERS OF THE ALLOY AN AQUEOUS PICKLINGSOLUTION CONSISTING ESSENTIALLY OF, BY WEIGHT, FROM 0.3% TO 3% HYDROGENFLUORIDE, NOT EXCEEDING 20% OF ACETIC ACID, AND 3% TO 30% AMMONIUMPERSULTFATE FOR A PERIOD OF TIME AT A TEMPERATURE OF FROM 70*F. TO120*F.